It's not a true dual band.
What happens when you press the key while the squelch is open? I would think that it transmits on the frequency that has the squelch open (receiving).
It can only work one frequency at a time. Although it can monitor two frequencies. Only actively receive on one at a time.
Well, that's sort of true. The Baofeng UV-5R only has one receiver. And it has only one bank of 128 memories.
In reality it can only monitor one frequency at a time. But when you enable the "dual watch" (TDR - menu item 7), what it does, it is switches rapidly between what shows in side A and what shows in side B of the display. And it does it fast enough that you don't really miss anything. It behaves as though it has two receivers, even though it really doesn't. However, when it's receiving a signal, it seems to suspend the TDR function while it has a signal acquired.
By the way, if you're listening to an FM broadcast station, and activity happens on either side A or side B, it suspends
the broadcast reception and switches to the active frequency. Once the signal goes away on the active frequency for
a few moments, the radio returns to FM broadcast listening.